This invention relates to jitter control in digital communications links.
More particularly, the invention relates to jitter control in a chain of asynchronous digital communications links.
Communications networks frequently include a chain of nodes N interconnected by links. One such arrangement is disclosed in International Publication No. WO 86/03639. Digital informaton starts at node N.sub.1 and is relayed, with or without modification to its content, successively by nodes N.sub.2, N.sub.3, . . . . In the case of the three typical adjacent nodes N.sub.i-1, N.sub.i, N.sub.i+1, information is transmitted at the rate of f.sub.i-1 bits per second from N.sub.i-1 to N.sub.i, at the rate f.sub.i from N.sub.i to N.sub.i+1. The rates . . . f.sub.i-1, f.sub.i, f.sub.i+1 . . . are nominally the same, but are dictated by separate clocks, respectively in nodes N.sub.i-1, N.sub.i, N.sub.i+1, and therefore differ from each other by unpredictable and varying, even if small amounts.
One known practice to avoid these variations is to operate such a chain synchronously without a local clock wherein the only autonomous clock is in N.sub.1 and the clocks in N.sub.2, N.sub.3, . . . are slaved to it. The slaving is typically done by a technique known as bit clock extraction whereby each node synchronises the rate and phase to the incoming bit stream. It then uses the same clock to form and transmit the outgoing stream. There is an ever increasing difficulty with this approach as the chain is made larger. Clock extraction is subject to a finite amount of jitter in phase and the jitter accumulates from node to node leading to a progressively increasing incidence of bit errors.
The use of independent clocks in the nodes is another known technique for overcoming the phase jitter problem. An extracted clock is still used on the receiving side of the node, but the transmission is by an independent clock and therefore not affected by the jitter in the reception. Such an arrangement is made possible by making all transmissions at a higher rate than would be necessary to transmit the actual data and by adding stuffing bits into the stream. The number of stuffing bits over a fixed interval of time can vary from link to link, the differences compensating for the differences in clock rates. By deletion or insertion of stuffing bits or groups of bits, the elastic buffers are not allowed to overflow or go empty.
The digital information is generally formatted, and generally into frames of fixed numbers of bits, the frames recurring at nominally fixed intervals. To allow for stuffing, the format must provide space for stuffing bits. Typically this can be the residual time interval between the end of data of one frame and the beginning of the next frame. Since the number of stuffing bits is variable, there is the further requirement that there be a definite indication of the start of the frame.